(1) In primary treatment, waste water is first
passed through bar screens, where large objects like sticks, plastic packets
and cans are removed. The waste water is passed through a grit chamber to
remove sand.

(2) The
sewage is then held temporarily in a sedimentation tank, where large, heavy
solids, faeces settle at the bottom by sedimentation. The debris that settles
at the bottom is called sludge.

(3) Oil,
grease, soaps, plastics and other lighter solids float over the surface. The
light floatable waste that rises to the top is called scum. The scum is
removed with a skimmer.

(4) Once settled and floating
materials are removed, the remaining liquid moves to the secondary treatment
process.

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